Winchester hard disk drives are well-known in the art. Typically, there are two or more recording magnetic surfaces. Data and servo (recording head positioning) information is recorded on the magnetic surface as digital pulses along concentric circles called tracks, starting from the inner radius and extending out to the outer radius. The tracks are bounded by two buffer zones located on the innermost radius and the outermost radius respectively.
Where there are multiple surfaces, one surface may be dedicated as a servo surface for controlling the flying magnetic heads. A dedicated servo surface allows correct head positioning as long as there are no mechanical or thermal effects which change the actual radius of a data track or create a tilt between the carriage of the heads and spindle of the disks. However, in low profile disk drive systems with fewer recording surfaces, the use of a dedicated servo surface further limits the data/servo efficiency. For example, in a single platter system with two recording surfaces, i.e. one data surface and one servo surface, the maximum theoretical data/servo efficiency is 50%. Similarly, in a dual platter system with four recording surfaces, i.e. one servo surface and three data surfaces, the maximum theoretical data/servo efficiency is 75%.
Alternatively, to avoid dedicating one recording surface entirely for servo fields, these servo fields can be dispersed between the data fields on all recording surfaces thereby freeing up an entire recording surface which increases the data/servo efficiency and improves tracking accuracy. This method of interleaving servo and data fields, commonly referred to as embedded servo, is now frequently used, especially for low profile hard disk drives having a minimal number of platters, necessary for fitting inside very compact portable personal computers.
In a conventional disk drive using a constant frequency recording method without a dedicated servo surface, all tracks from the outside radius to the inside radius hold the same amount of data, i.e. the same number of data sectors. Since the recording density of the outside radius is lower than that of the recording density of the inside radius, there is inefficient use of the outer tracks. With this technique, the data sectors are distributed evenly and separated by the servo fields, with servo fields radially aligned such that no discontinuity is encountered between tracks. FIG. 1 shows one example of a disk with constant frequency recording and interleaved servo and data. T.sub.1, T.sub.2 and T.sub.3 are three tracks with different radii, but with an identical number and distribution of data sectors and servo fields. A first set of servo fields S.sub.10, S.sub.20 & S.sub.30, a first set of data sectors A.sub.1, B.sub.1 & C.sub.1, a second set of servo fields S.sub.11, S.sub.21 & S.sub.31, a second set of data sectors A.sub.2, B.sub.2 & C.sub.2, and a third set of servo fields S.sub.12, S.sub.22 & S.sub.32 all line up radially.
One method for optimizing the use of the outer tracks is constant density recording which maintains the recording density of the innermost track throughout all the tracks on the disk surface. For example, if the inside radius of a 3.5 inch disk recording area is 0.9 inch and the outside radius is 1.71 inches, with constant density recording, the outermost track has 1.9 times the capacity of the innermost track, with a corresponding theoretical disk capacity increase of 45%. As a bonus, the data rate is also improved 1.9 times at the outermost track.
In most computer memory systems, data is packaged into data sectors, typically 512 bytes long each. Therefore, with constant density recording, the number of data sectors increases progressively in integer steps from the inside radius to the outside radius. As a result, there are multiple zones, each zone having the same number sectors on every track within the zone to approximate a constant density recording. This technique is referred to as Zone Data Recording (ZDR). FIG. 2 shows three tracks, T.sub.1, T.sub.2 and T.sub.3, from three different zones, each representative of tracks having the same number of data sectors recorded at the same frequency within each respective zone.
In the above example, with 2000 tracks per inch (tpi) and 50 sectors at the innermost track. The number of tracks in a zone is tpi times the inside radius, divided by the number of sectors in the innermost track, or 2000.times.0.9/50=36. The number of zones on a disk is the radial span of the recording area (1.71.times.0.9) times the tpi, divided by the number of tracks in a zone, or 0.81.times.2000/36=45. Forty-five zones yields a theoretical efficiency of 44%, which compares fairly well with the previously calculated 45% capacity increase via constant density recording.
However, each zone requires different recording reference frequencies, filter parameters and data separation, which make it prohibitive from the data retrieval/storage management standpoint. Therefore, in practice, the number of zones is limited. Using the above example, with 5 zones instead of 45, the number of data sectors incremented between adjacent zones becomes 45/5=9. The number of tracks in a zone being 36.times.9=324. This yields an actual capacity increase of 36%, which is still very attractive.
Unfortunately, using the first ZDR method described above in combination with embedded servo fields introduces a new problem. Since entire data sectors (typically 512 bytes long) have to be used, with frequencies changing between zones, the number of data sectors increasing towards the outside zones, and with servo field and data sector boundaries only matching within each zone, there is now a discontinuity of servo fields between zones. With this misalignment of servo fields at the zone boundaries, when a magnetic recording head crosses a zone boundary, there is no servo information available until the recording head is almost on a track. Hence track acquisitions in seek operations are not smooth and if the tracks on the zone boundary happen to be on a final approach of a seek, the settling or deceleration of the recording head will not be consistent.
FIG. 2 illustrates the above described seek problem. Tracks T.sub.1, T.sub.2 and T.sub.3 of different zones, now have different recording frequencies, resulting in the misalignment of servo field boundaries across zone boundaries. A first set of servo fields S.sub.10, S.sub.20, & S.sub.30 of different zones start out aligned along one servo sector extending radially outward from the center of the disk. However, after a first set of data sectors A.sub.1, B.sub.1 & C.sub.1, a second set of servo fields S.sub.11 ', S.sub.21 '& S.sub.31 is radially misaligned relative to each other. The same is true of subsequent sets of servo fields such as 8.sub.12 ', S.sub.22 '& S.sub.32.
A further enhancement of the above described ZDR method addresses the problem of misalignment of servo fields and is called the Split (Data) Field ZDR which is illustrated in FIG. 3. Here, all servo sectors are lined up radially with the same frequency, and the continuity and servo automatic gain control (AGC) can be preserved. However, the data sectors and the servo fields boundaries do not line up anymore and there is a price to pay. Now, an added data Phase Locked Loop (PLO) sync field and an additional data marker byte a pad is necessary because of data sector splitting, and this is typically in the order of 12-14 bytes in a 512 sector.
Within a zone, there may be variations of servo/data sector patterns, and the number of variations will multiply N-fold for N zones. The efficient handling of ZDR data requires setting up look-up tables for these variations. In the example above, with 5 zones and 512 byte data sectors, there are about 50 sectors per track multiplied by five zones, and multiplied by at least three bytes per table entry, yielding a table size of at least 750 bytes.
In addition to the above memory overhead, the disk controller is now burdened with a more complex sequence of events to read data sectors that have been split into two portions by servo sectors. A typical read sequence of a split data sector includes the following steps:
a) Read ID sync and sync byte. PA1 b) Read ID and CRCC. PA1 c) Read prescribed number of data bytes in first portion of the split data sector. PA1 d) Store data in buffer and pause. PA1 e) Wait until a servo field is read. PA1 f) Read the second data sync field. PA1 g) Read the remainder of prescribed data bytes of second portion of the split data sector. PA1 h) Read ECC, store data, and report if the data is good.
In addition to the above described overheads of extra tables and complex operating sequences, prior art split field implementions have another disadvantage of being unable to split a data sector within its header information area. This is because typically the split data information is stored within the I.D. field of the header information area which must be retrieved before the controller knows where the split is located. As such, prior art split field implementation can only split the data sector at a location at least one data byte into the data field, in order for the data byte based counter to operate.
In the example of a Prior Art Split (Data) Field ZDR implementation illustrated in FIG. 3, tracks T.sub.1, T.sub.2 and T.sub.3 are representative of three different frequency zones. 0f particular interest is track T.sub.2 where data sector B.sub.2 is split into two portions B.sub.2a and B.sub.2b. Note that the sets of servo fields S.sub.10, S.sub.20 & S.sub.30, S.sub.11, S.sub.21 & S.sub.31, and S.sub.12, S.sub.22 & S.sub.32 of all three zones are now lined up radially just as in the constant frequency method illustrated in FIG. 1.
FIG. 10 shows a detailed layout of portions of a Prior Art split data sector comprising a header information area and a first data information area. The first data information area is followed by a similar second data information area (not shown).
In a conventional computer system, most of the disk drive hardware specific electronics is on a disk controller card. The host computer has no need and hence no knowledge of the actual physical location of the data and servo sectors on the disk drive(s). All the host computer and its operating system needs to know is the number of disk drive(s) available and the number of data sectors available on each drive(s).
In an IBM AT type computer system, the Disk Operating System (DOS) keeps track of where all the data reside within the available data sectors which are identified by their DOS data sector numbers. The host computer transfers data to and from the disk drive using I/O instructions via the Basic Input/Output System (BIOS) and the intelligent disk controller hardware, which are responsible for translating each DOS data sector number into a physical data sector location and retrieving the required data. The actual physical location and form of each individual data sector is transparent to the host computer using DOS. This gives disk drive manufacturers considerable latitude in designing the physical layout of the data and servo sectors, and implementing various schemes for increasing the data capacity of the disk drive including Split Field ZDR.